From Permanent Legal Immigration to the United States: Policy Overview (R42866, May 11, 2018):
Four major principles currently underlie U.S. policy on legal permanent immigration: the reunification of families, the admission of immigrants with needed skills, the protection of refugees and asylees, and the diversity of immigrants by country of origin. These principles are embodied in the Immigration and Nationality Act (INA) and are reflected in different components of permanent immigration. Family reunification occurs primarily through family-sponsored immigration. U.S. labor market contribution occurs through employment-based immigration. Humanitarian assistance occurs primarily through the U.S. refugee and asylee programs. Origin country diversity is addressed through the Diversity Immigrant Visa.
The pool of people eligible to immigrate to the United States as LPRs each year typically exceeds numerical limits established by the INA for most immigrant pathways. In an effort to process the demand for LPR visas fairly and in accordance with the national interest, the INA imposes a complex set of numerical limits and preference categories within major immigrant pathways that admit LPRs to the United States on the basis of family relationships, needed skills, and geographic diversity.
The INA limits worldwide permanent immigration to 675,000 persons annually: 480,000 family sponsored immigrants, made up of family-sponsored immediate relatives of U.S. citizens (“immediate relatives”), and a set of ordered family-sponsored preference immigrants (“preference immigrants”); 140,000 employment-based immigrants; and 55,000 diversity visa immigrants. This worldwide limit, however, is referred to as a “permeable cap,” because certain categories of LPRs are not subject to numerical limitations. These include immediate relatives of U.S. citizens within the INA’s family-sponsored immigration provisions, as well as refugees whose number is determined by the President in consultation with Congress. In addition, the number of persons granted asylum is not numerically constrained. Consequently, the number of persons receiving LPR status each year regularly exceeds the INA’s statutory worldwide level for permanent immigration.
The INA further specifies that countries are held to a numerical limit of 7% of the annual worldwide level of family-sponsored and employment-based immigrants, known as the per country limit or country cap. The cap is intended to prevent one or just a few countries from dominating immigrant flows.